The present invention relates to a technique effective to be applied to a clock synchronization technique in a communication system, further in the case of synchronizing two or more semiconductor integrated circuits operated by different clock signals, respectively. For instance, the present invention relates to a technique effective to be used in a portable phone including an LSI (baseband LSI) performing modulation and demodulation processing of voice data and a logic LSI (application processor) compressing and decompressing voice data.
In recent years, portable phones having a videophone function have been developed. A portable phone having a videophone function has a baseband LSI performing baseband processing such as modulation and demodulation of voice data, a logic LSI called an application processor compressing and decompressing voice data and moving image data, and a radiofrequency LSI performing up-convert of a transmission signal and down-convert of a reception signal.
In the case of performing communication between portable phones, they must be synchronized with each other. A portable phone is operated by a clock signal (hereinafter, simply called a clock) generated by its own oscillator. Due to a variation in frequency of the respective oscillators and a frequency drift with temperature change, a clock frequency is different in each portable phone. The Doppler shift by fast movement of the user of a portable phone causes a difference between clocks. Correct synchronization cannot be obtained. Accordingly, in a portable phone system, each of portable phones has an AFC (Automatic Frequency Correction) function correcting a clock frequency of the portable phone based on time information of a master clock of a base station included in data received from the base station to the portable phone, ensuring synchronization between the portable phones.